mjhjmtu

I want to prepend something to each line of output in a script, for every command.

I was thinking of doing something like this:

rm foo
mkfifo foo

exec 3<>foo

cat <&3 | while read line; do
 if [[ -n "$line" ]]; then
 echo " [prepend] $line";
 fi
done &

echo "foo" >&3
echo "bar" >&3
echo "baz" >&3

basically for all commands I want to prepend something to each line of the output. My code above is fairly bogus, but I don't quite know how to do it, it's something like the above but not quite.

Assuming script produces:

L1
L2

L4
L5

then the following command

script | sed 's/^(.+)/ [prepend] 1/'

prepends " [prepend] " to each non-empty line:

 [prepend] L1
 [prepend] L2

 [prepend] L4
 [prepend] L5

You may want to look at the DEBUG trap in bash. From man builtins:

If a sigspec is DEBUG, the command arg is executed before every simple command,
for command, case command, select command, every arithmetic for command, and
before the first command executes in a shell function (see SHELL GRAMMAR
above). Refer to the description of the extdebug option to the shopt builtin
for details of its effect on the DEBUG trap. If a sigspec is RETURN, the com‐
mand arg is executed each time a shell function or a script executed with the .
or source builtins finishes executing.

So, you could set up a debug function like this. Since it runs before commands you could use it to prepend to your output.

#!/bin/bash

debug() 
 : # echo or other commands here


trap debug DEBUG

# Commands here

This exact code seems to do what I want, but not sure how safe it is:

rm foo
mkfifo foo

exec 3<>foo

(
 cat <&3 | while read line; do
 if [[ -n "$line" ]]; then
 echo " [prepend] $line";
 fi
 done &
 )

echo "" >&3;
echo "" >&3;
echo "foo" >&3
echo "bar" >&3
echo "baz" >&3


pkill -P $$